1. Field of the Invention
The present invention relates to a drive switching mechanism for switching transmission of a drive and to a feeding device including the drive switching mechanism.
2. Description of the Related Art
In a recording apparatus, such as a printer, a copier, or a facsimile machine, or a reading apparatus, such as a scanner, to form an image on a sheet material (a recording medium (e.g., paper) or a document) or read a formed image, sheet materials stacked on a sheet stacking portion are separated one by one. A separated sheet material is conveyed by a conveying portion to a recording portion or a reading portion. After the completion of forming or reading the image, the sheet material is ejected from the apparatus by an ejecting portion. Many recording apparatuses or reading apparatuses that switch a drive from a single driving source using a drive switching mechanism, separate sheets, and convey a separated sheet to achieve a more inexpensive configuration are developed nowadays. Examples of such a known configuration are described in detail in Japanese Patent Laid-Open Nos. 2004-075277 and 2005-351435.
Japanese Patent Laid-Open No. 2004-075277 discloses an apparatus having a configuration in which a feeding portion that includes a drive switching mechanism is driven by a driving force transmitted from a driving source through a conveying roller. The drive switching mechanism includes a sun gear and two planetary gears engaging with the sun gear. A first planetary gear drives a feeding roller gear in a direction in which sheets are conveyed through a control gear by the conveying roller rotating in a forward direction. A second planetary gear directly drives the feeding roller gear in the direction in which sheets are conveyed by the conveying roller rotating in a backward direction. In this drive switching mechanism, the feeding roller is driven by the conveying roller backwardly rotating to start separating recording sheets and feeding a separated recording sheet. The front end of the separated recording sheet comes into contact with a nip portion formed between the conveying roller backwardly rotating and a pinch roller being pressed into contact with and following the conveying roller, and skew of the front end is corrected along the nip line. After that, the recording sheet is conveyed to the recording portion by the conveying roller forwardly rotating, and an image is formed on the recording sheet. Then, the sheet is ejected from the apparatus.
Japanese Patent Laid-Open No. 2005-351435 discloses an apparatus having another configuration in which a feeding portion that includes a drive switching mechanism is driven by a driving force transmitted from a driving source through a conveying roller. The drive switching mechanism includes a sun gear and two planetary gears engaging with the sun gear. A first planetary gear drives a feeding roller gear in a direction in which sheets are conveyed through a control gear by the conveying roller rotating in a forward direction. The first planetary gear also includes a one-way clutch. Thus, during forward rotation of the conveying roller the driving force of the planetary gear can be transmitted, whereas during backward rotation thereof the planetary gear idles and the driving force is not transmitted. A second planetary gear directly drives the feeding roller gear in the direction in which sheets are conveyed by the conveying roller rotating in a backward direction. The first and second planetary gears are in a stand-by state in which they do not carry out feeding in normal times by a stopper that restricts swinging. When a carriage is moved to a position where the stopper is released and the conveying roller is forwardly driven, the control gear is engaged with a planetary gear and the feeding roller is driven in the direction in which sheets are conveyed. The control gear has a groove for engaging with an arm of the first gear. During feeding, swinging of the first planetary gear is restricted, and the first planetary gear maintains a state in which it always engages with the control gear. Accordingly, even if the planetary gears are driven in the forward or backward direction during feeding of recording sheets, a drive can be transmitted by the planetary gears without delay. This enables correction of skew of the front end of a recording sheet along a nip after the recording sheet is transported to the conveying roller backwardly rotating and also enables correction of skew of the front end of a recording sheet along a nip after the recording sheet is temporarily engaged in the conveying roller forwardly rotating and then is reversely moved to the roller nip position by the conveying roller backwardly rotating. Additionally, feeding without the above correction of skew of the front end can also be carried out, so various kinds of feeding can be performed depending on the type of a sheet.
The drive switching mechanism described in Japanese Patent Laid-Open No. 2004-075277 feeds a sheet by backwardly rotating the conveying roller on all occasions. Because it is necessary to forwardly rotate the conveying roller until printing on a previous page and ejecting the sheet are completed, a next page cannot be fed until then. That is, it is difficult to improve throughput by feeding a next page during ejection of a previous page.
The drive switching mechanism described in Japanese Patent Laid-Open No. 2005-351435 starts feeding by forwardly rotating the conveying roller. It is possible to feed a next page during ejection of a previous page (forward rotation of the conveying roller). Due to the groove of the control gear, the first planetary gear always engages with the control gear during feeding. However, after feeding, it is necessary to release engagement between the planetary gear and the control gear in order to avoid further feeding resulting from forward rotation of the conveying roller in subsequent conveying. To this end, the conveying roller needs rotating slightly in the backward direction to release the engagement between the planetary gear and the control gear after the completion of feeding. Although a time required for backward rotation for each page is rather short, in these days, when further improvement in throughput is being demanded, the time required to release the engagement cannot be ignored.
In terms of improvement in throughput, backward rotation of the conveying roller during feeding of a recording sheet is preferably minimized except for backward movement necessary for skew correction of a sheet. Accordingly, a drive switching mechanism that completes feeding using forward rotation alone is desired.